In a conventional system called a top-view or surround-view system that displays the surroundings of a vehicle as viewed from above, cameras mounted on the outer periphery of the vehicle take images of the road surface obliquely from sides, a position (a height and a direction with respect to the cameras) of the plane of the road surface is provisionally determined, and a road image of the entire external surroundings of the vehicle is formed by combining the respective images from the cameras as a projected image on the assumption that the road surface is a flat surface according to the captured images.
Shifts and the like from ideal set values based on actual installation are caused by manufacturing and installation. Therefore, at the stage of the vehicle manufacturing, internal parameters (a camera focal length, a distortion amount, and an optical axis shift) for each camera and external parameters (an installation height from the road surface, a direction/pitch and a yaw angle of the optical axis with respect to the road surface, and turning/rotation with respect to the road surface) for each camera are calibrated, so that the installation errors are absorbed, and combined images of the respective cameras mounted on a plurality of positions are continuously displayed in an ideal manner.
However, even if shifts from the ideal set values of the installation height and the orientation of the cameras constituting the system with respect to the road surface are calibrated, the relative positional relationship with the road surface might change due to various factors such as the load amount and balance on the vehicle, physical impact, and the like, and the calibrated values might change accordingly.
To counter these shifts of the installation height and orientation and changes in the features unique to the cameras, special calibration charts are provided around the vehicle in a special place for the time of the vehicle manufacturing or maintenance of the completed vehicle, and calibration for parameters that are the factors of the changes is performed with the use of target patterns and the like on those charts.
For example, Patent Document 1 discloses a technique by which an image shift is detected with a pair of front and rear cameras. Further, Patent Document 2 discloses a technique by which calibration of the front camera is performed using the vanishing point.
On the other hand, if there is a load on the vehicle or a slope or a step on the road surface, an image obtained as a result of integral overhead-view transform still has a shift in the original state, even though ideal projected images of the road surface are formed by the respective cameras. If the situation changes, the road image also changes. Therefore, it is not realistic to perform calibration at the place of the manufacturer or a service station each time as in the case of maintenance while the shift still remains.